White LEDs have been a popular choice for use in backlighting, torch and camera flash applications, and manufacturers of consumer electronic device have embraced charge drivers to power white LEDs in mobile systems that run on batteries such as lithium-ion or polymer batteries. In such applications, white light illumination needs to be bright and with increases in the resolution of images, which now exceed mega pixels per image, white light illumination needs to be even brighter. Thus, in most applications, several white LEDs are combined to operate together in order to create brighter illumination. However, the intensity of white LEDs may vary and their color may not be homogeneously white when combined. This is because the forward current characteristics of white LEDs are not exactly similar in all of them.
During flash operations, for instance, white LEDs may require at least 2 A flash current to create sufficiently bright illumination and a current drain from the battery that is limited to nearly 1 A. Lithium-ion or polymer batteries produce a limited current peak of about 1.5-2 A during flash operations and such currents discharges the batteries fast. The current grows proportionally with increases in the voltage and for white LEDs there is therefore a need for higher voltage. However, as current is drawn from the batteries their internal resistance increases and with it their voltage drops. To operate, white LEDs may require a voltage above the 3.0V level to maintain sufficient light intensity and substantially avoid remaining dark. A fully charged lithium-ion or polymer battery provides a typical output of 4.2V, which quickly drops to a 3.0V level as the battery discharges. If white LEDs are operated directly from the battery and the voltage drops, the light intensities decrease and the differences in color become stronger. Therefore, more accurate current control and a minimum level of operating voltage are typically required.
Boost converters and energy storage devices have been developed to support such current requirement by providing a source of instant high energy. For example, boost converters can increase the voltage and reduce the number of cells where it is otherwise impractical to stack batteries. Generally, a boost converter is a voltage step-up converter that is often regarded as a switching mode power supply. A common boost converter circuit includes two or more switches (such as transistor and diode) and a battery and one or more filters made of inductor and capacitor combinations. FIG. 1A illustrates an exemplary boost converter circuit. Thus, although boost converters may be efficient, these circuits are typically external to the flash current source and include bulky components, such as coils, that increase the overall cost and size of the circuit board. Additionally, boost converters (or switching regulators or charge pumps) are not based on charge storage and, depending on the voltage ratio between the forward voltage (VF) of the white LEDs and the voltage supplied from an associated battery, they may require such battery to supply twice the flash current. FIG. 1B is a graph representation of boost converter behavior, including a graph of the flash current and boost converter output voltage. As shown, the current overshoots in a flash state and declines once the flash state ends, requiring supply in excess of 3 A during the flash state.
Unlike the boost converters, energy storage devices are based on charge storage and they are used as a power source for the white LEDs instead of the battery (the battery is, in turn, used as the power source for the charge storage). The charge can be stored by draining current from the battery over time. A type of high-energy storage device that is based on charge storage with high-power discharge is known as the “supercapacitor.” Supercapacitors are charge storage devices configured with two capacitor components in series and operating as an energy reservoir capable of providing burst power needed in high power applications such as flash and torch lighting. Supercapacitors are designed to be charged and recharged, repeatedly and to provide instantaneous high discharge currents with rapid recharge between discharge operations.
FIG. 2 illustrates a charge circuit with a supercapacitor used in a flash application. Another circuit may include a combination of the supercapacitor and boost converter, as shown in FIG. 3. One such supercapacitor, provided by CAP-XX, Inc. of Sydney, Australia, exhibits high capacitance of 10 mF to 2.8 F and yields high energy density. Accordingly, a supercapacitor is used in this circuit to store the amount of charge needed for providing a flash energy pulse (i.e., the burst power needed by the white LEDs). The charge can be stored in the supercapacitor by draining the current from the battery over time, and if the current is too low the charge time will be long and perhaps longer than is reasonable for rapid succession of flash operations. Indeed, it take high current to rapidly re/charge the supercapacitor, and with battery power depleted after one or more flash operations the available charge current can be rather small and result in relatively long charge time.
As can be seen, in both FIGS. 2 and 3, the supercapacitor includes two capacitors in series with a center tap between them. Inherently, this design causes charge imbalance and for this reason there is a pair of charge balancing resistors to equalize the voltage across both capacitors and, in turn, create a charge balance. The supercapacitor also suffers from apparent leakage that, in time, causes charge drain.